Condensation-Incompetent Ketosynthase Inhibits trans-Acyltransferase Activity.

Nonelongating modules with condensation-incompetent ketosynthase (KS) are frequently found in many trans-acyltransferase polyketide synthases ( trans-AT PKS). KS catalyzes translocation of carbon chain without decarboxylative condensation. Unlike typical elongating modules where malonylation of acyl carrier protein (ACP) precedes elongation, the malonylation of ACP downstream of KS is assumed to be prevented. In this study, the regulation mechanism(s) of ACP malonylation in a non-elongating module of difficidin biosynthase was investigated. In vitro reconstitution, protein mass spectrometry, and enzyme kinetics demonstrated that KS controls the pathway by inhibiting the trans-AT activity. Protein-protein interactions of the surrounding domains also contribute to the regulation. Enzyme kinetics further identified the DfnKS5 as an allosteric inhibitor of trans-AT. The principle and knowledge discovered from this study will enhance the understanding of this unusual PKS system.